This invention relates to a core structure particularly for use in articles such as boats in conjunction with composite materials such as fiberglass reinforced plastic.
Presently, most boats of the pleasure craft variety and numerous other articles are made from fiberglass reinforced plastic (FRP) materials. In fabricating such structures, it is often necessary to provide stiffening members, webs, bulkheads, and other members in order to provide the desirable structural characteristics. Often such features are of a thickness which is greater than that which can be conveniently formed from FRP materials. Typically, thick sections of FRP material require many layers of fiberglass mat and resin which are normally laid up by hand. This process is time consuming and costly since it involves a great deal of direct labor. In addition, extremely thick sections of FRP may have an unacceptably high weight for a given device. Moreover, the above problems of developing thick sections are not confined to FRP materials. It is also difficult to provide such thick sections using numerous other types of structural materials.
In order to overcome the disadvantages associated with thick sections of structural materials, such as FRP, core structures of a desired thickness are often used which are covered by skin layers. Many present boats and other articles have cores made from slices of lightweight balsa wood material. Balsa wood is frequently cut into flat plate-like slices and may be formed from multiple pieces which are held together by a fabric mat. Skin layers of FRP are applied to both sides of the balsa core to encapsulate it. Although balsa cores provide the desirable section thickness and perform satisfactorily in many respects, they have numerous disadvantages. In the event of a crack or hole in the skin layers encapsulating the core, water or other liquids can seep into the core. If this occurs, the balsa wood core may become saturated and will not readily dry out since it is not ordinarily exposed to freely circulating air. Soaking of a balsa core causes it to lose its structural characteristics and buoyancy. Another disadvantage associated with balsa wood cores is that it does not readily become wetted by the liquid FRP resin which is applied to the surfaces of the core, and consequently, a secure bond between the core and the outer skin layer is not provided. Such lack of interengagement reduces the structural integrity of the core structure. As a means of overcoming the above-mentioned lack of structural engagement between the skin and balsa wood core, the curing time of the FRP resin may be slowed to allow it to "wet" the balsa surface. This approach, however, increases cycle time leading to a production cost penalty. Balsa cores have the additional disadvantage that balsa wood material is fairly costly. Various alternate core materials have been employed with varying degrees of success. For example, plywood and aluminum honeycomb materials have been used. Such materials, however, impose cost, manufacturing or performance drawbacks.